When encapsulation molding of integral circuits, etc. (hereinafter referred to as IC/LSIs) with a thermosetting resin such as an epoxy resin is continued for a long time, cases not infrequently occur in which the inner surface of the mold is stained, and continuation of molding with such a stained mold results in contamination of the surface of the molded products or adhesion of the molded product to the mold surface, making further shots impossible. The mold should be cleaned periodically, which is practiced by making a few shots with a mold cleaning resin per several hundreds of shots with a molding material thereby to remove deposited stubborn stain.
Mold cleaning using a conventional cleaning resin is carried out by a method comprising setting a lead frame to be encapsulated or a dummy lead frame made of cheap metal or paper in a mold and filling the cavity with a cleaning resin compound from a transfer pot or a method comprising placing a cleaning resin compound on the parting face, clamping the mold, and heating.
However, these mold cleaning methods have the following problems. According to the method comprising setting a lead frame to be encapsulated or a dummy lead frame in a mold and filling the cavity with a cleaning resin compound from a pot, the lead frame or the dummy lead frame used for cleaning is encapsulated in the cleaning resin and becomes non-reusable. That is, a new lead frame or a new dummy lead frame should be used for each cleaning shot, which is uneconomical.
Another problem is that, when a lead frame or a metal-made dummy lead frame is used, the molded part resulting from the cleaning operation must be separated into the resin part and the metal part before disposal as industrial waste. Besides, the separation is difficult.
In order to overcome these problems, a compression type cleaning method is also practiced, which is carried out by heat melting and curing a cleaning resin compound between upper and bottom mold without using a lead frame or a dummy lead frame. This method is disadvantageous in that placement of the cleaning resin needs time and that occurrence of resin leakage or chipping makes the cleaning cycle time longer, which results in a totally increased cleaning time.
To address the above-mentioned problems, JP-A-7-304044 proposes a cleaning method using a cleaning material prepared by preliminarily molding a cleaning resin on a base sheet or tape.
The cleaning material of JP-A-7-304044 is prepared by preliminarily molding a cleaning resin, such as an epoxy resin or a melamine resin, on a sheet or tape form base by low-temperature pressure forming of the resin powder (tableting) or fusion bonding. However, tableting the cleaning resin powder onto the base to make an integral cleaning material is difficult. Moreover, the tableting, even if carried off, is accompanied by dusting, and the formed resin powder tends to lose the shape during transportation. Where fusion bonding is adopted to preliminarily mold the cleaning resin on the base, it is also difficult to carry out. Even if the resin is successfully fusion bonded to the base, flowability of the cleaning resin is reduced by the thermal history given to the resin during fusion bonding. It follows that the cleaning resin fails to fill every part of the mold only to result in insufficient cleaning. Making through-holes in the base, as suggested to improve flowability of the cleaning resin to the cabity, leads to reduction in strength of the base.
In addition, diversification of electronic devices including IC packages has diversified molding dies therefor. This has made it very difficult to dispose the cleaning resin in accurately right position corresponding to the cavity. That is, the cleaning resin has poor workability in mold cleaning.